GE GFK-1541B manual Multiple Sntp Servers, Loss or Absence of Sntp Timing Signals

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Multiple SNTP Servers

To guard against loss of SNTP timing messages, multiple SNTP time servers can be tracked on a network. An Ethernet Interface can maintain timing information from up to four total SNTP time servers at a time. Each server assigns a stratum number that determines its priority. The message from the server with the lowest stratum number is used by the Ethernet Interface until communication with that server is lost. Then the server with the next lowest stratum number becomes the server of choice and the Ethernet Interface synchronizes to it if it receives two of its timing messages within a 150-second period. A server is considered "lost" if more than 150 seconds elapse between timing messages.

The Station Manager command, CHSNTP, can be used to override the Ethernet Interface’s automatic server selection strategy and identify specific server(s) to be used. For more information on using the Station Manager, refer to GFK-1186, TCP/IP Ethernet Communications for the Series 90 PLC Station Manager Manual.

Loss or Absence of SNTP Timing Signals

If an Ethernet Interface is configured for SNTP, but does not receive two timing messages from an SNTP network time server within a 150-second period, the following will happen:

ƒA fault entry will be placed in the PLC Fault Table.

ƒA fault entry will be placed in the Ethernet Interface's exception log. This log can be read by using the GE Fanuc Station Manager. Details can be found in GFK-1186, TCP/IP Ethernet Communications for the Series 90 PLC Station Manager Manual.

ƒThe Status word within a consumed exchange will indicate new data with a value of 3, instead of the normal 1 value, indicating that SNTP is selected, but the Ethernet Interface is not synchronized to an SNTP server. This Status word value can be obtained from the PLC register configured for the particular exchange.

Note

The SNTP error condition is considered the least important of all possible error codes. Therefore, if another error condition exists, its status code will appear in the Status word instead of the SNTP error code.

Upon loss or absence of synchronization, the Ethernet Interface’s built-in clock will operate as follows:

ƒIf the Ethernet Interface, after its last power-up/restart cycle, has never received an SNTP server's timing message, it will continue to use the PLC CPU's local clock value that it received at power-up/restart for its time base.

ƒIf the Ethernet Interface has, at some point, been synchronized to an SNTP server but lost its signal, it will use the most recently received SNTP time message as its time base.

The Ethernet Interface will continue supplying time values to the PLC CPU for timestamping, while it "listens" for SNTP timing messages from the network. If SNTP messages are received later, the Ethernet Interface will then synchronize to them.

5-26 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B